The present invention relates to a tread for a pneumatic radial tire.
The tread portion of a pneumatic tire generally comprises an elastomeric material having a plurality of grooves therein defining ground engaging rubber elements. The particular size and shape of these elements contributes significantly to the overall performance of the tire. Tire treads are generally designed to provide a particular performance, such as for instance winter, high traction or high speed performance. Thus obtaining of one particular performance characteristic is often at odds with obtaining another one.
Hydroplaning of tires on wet pavement has long been a problem in the prior art.
Various tire designs, adapted to channel water away from the tire, and thereby maintain rubber contact with the road have been created. Although prior art rain tire designs, such as described e.g. in EP-A-508 090, have improved wet traction, it is a continuing goal in the art to further improve such.
French patent 1 214 717 teaches a way to improve road adherence by providing the tread pattern with raised block elements. The raised elements are described as acting as elastic, heavily compressed projections entering the small holes in the road surface and improving thereby the grip of the tire. U.S. Pat. No. 5,343,918 discloses a tire tread with at least one circumferentially extending row of high stiffness blocks and one of low stiffness blocks.
The low stiffness blocks have a radial height between 4% and 10% superior to the radial height of the high stiffness blocks. The object is to obtain uniform handling response and to improve grip.
In French Patent FR782332A teaches that the tread height near one edge of a centermost groove extends above the average tread height to improve traction.
In the Japanese Patent Abstract JP05338412A to Sumitomo Rubber Ind LTD, it teaches the groove wall edges can be acute angles to improve the useful life of the tire.
In the most relevant document EP-A-0-508 090 the features recited in the preamble of the claim are disclosed. Those features after the preamble being the novel features not found in the art in the combination claimed. The dependent claims representing features of the various preferred embodiments of the invention.
It is an object of the present invention to provide a pneumatic tire having improved wet traction while maintaining good handling, noise and irregular wear characteristics.
Other objects of the invention will be apparent from the following description and claims.
A tread 2 for a pneumatic tire 1 has elastomeric material. The tread 2 has a central portion and two shoulders 3, 4. The tread defines ribs or rows of lugs.
The central portion includes three laterally spaced circumferentially extending grooves 7, 8, 9, defining two circumferentially extending center ribs 22, 23 having an axial width RW ranging from 5% to 25% of the tread width TW of the ground engaging tread portion.
The centermost circumferentially extending groove 8 has on both sides tapered groove sidewalls which form with a radial plane NP acute angles xcex11 and xcex12 ranging between 1 and 10 degrees, preferably between 2 and 7 degrees, more preferably between 3 and 5 degrees. The tread height near both edges P0,P2 of the centermost groove extending radially above the average tread height a maximum extra tread height comprised of 0.3 mm and 1.7 mm most preferably between 0.8 and 1.2 mm. This extra tread height diminishes progressively from the groove edges towards the respective nearest shoulder of the tire over a distance not exceeding 10% of the tread width.
Preferably the circumferentially extending center ribs 22, 23 are cut by snipes 14 and may include blind grooves 52, 53.
The width of the circumferentially extending centermost groove at 8, 53, 68, 77 the tread surface is between 5 and 20 mm and the groove is spaced from the equatorial plane a distance equaling at most 25% of the treadwidth TW.
The tread 2 is delimited by a first and a second shoulder 3, 4. The central portion includes three laterally spaced circumferentially extending grooves defining two circumferentially extending center ribs having an axial width RW ranging from about 5% to 25% of the treadwidth TW of the ground engaging tread portion.
The centermost circumferentially extending groove has on both sides tapered groove sidewalls, which form with a plane parallel to the equatorial plane acute angles xcex11 and xcex12 ranging between 1 and 10 degrees.
In an axial cross section of the tread 2, the radially outer tread surface, on a first side and second side of the centermost groove, has at least a first and at least a second radius of curvature R1 and R2 respectively. The first radius of curvature R1 has its center C1 radially outwardly of the tread surface, in a plane parallel to the equatorial plane and located between the first side of the center groove and the fist shoulder of the tire. R1 extends from a point P0 on the edge the groove wall forms with the tread surface to a point P1 located a distance between 5% and 15% of the treadwidth, defining thereby a first radially outer concave tread surface.
The second radius of curvature R2, has its center C2 radially outwardly of the tread surface, in a plane parallel to the equatorial plane and located between the second side of the center groove and the second shoulder of the tire. R2 extends from a point P2 on the edge the groove wall forms with the tread surface to a point P3 located a distance between 5% and 15% of the treadwidth, defining thereby a second radially outer concave tread surface.
The centers of curvature C1 and C2 lie preferably in a plane located from the respective edges of the centermost groove they define, a distance ranging between 5% and 10% of the treadwidth. The point P1 is spaced from P0 a distance of about 7% of the tread width and the point P3 is spaced from P2 a distance of about 7% of the treadwidth.
xe2x80x9cAspect Ratioxe2x80x9d of the tire means the ratio of its section height to its section width.
xe2x80x9cBeadxe2x80x9d means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chaffers, to fit the design rim.
xe2x80x9cCarcassxe2x80x9d means the tire structure apart from the belt structure, tread, undertread, and sidewall rubber over the plies, but including the beads.
xe2x80x9cCrownxe2x80x9d refers to the circumferentially outermost portion of the carcass substantially within the width limits of the tread.
xe2x80x9cDesign rimxe2x80x9d means a rim having a specified Configuration and width.
xe2x80x9cDirectional treadxe2x80x9d refers to a tread design which has a preferred direction of rotation in the forward direction of travel.
xe2x80x9cEquatorial plane (EP)xe2x80x9d means the plane Perpendicular to the tire""s axis of rotation and passing through the center of its tread.
xe2x80x9cFootprintxe2x80x9d means the contact patch or area of contact of the tire tread with a flat surface at zero speed and under design load and pressure.
xe2x80x9cFootprint Net-to-grossxe2x80x9d refers to the actual footprint of a deflected tire and is the ratio of the ground contacting surface area of the tread to the total tread footprint area including the groove void area.
xe2x80x9cGroovexe2x80x9d means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight, curved, or zig-zag manner. Grooves ordinarily remain open in the tire footprint. Circumferentially and laterally extending grooves sometimes have common portions and may be sub-classified as xe2x80x9cwidexe2x80x9d or xe2x80x9cnarrowxe2x80x9d. Grooves may be of varying depths in a tire. If such narrow or wide grooves are of substantially reduced depth as compared to wide circumferential grooves which they interconnect, they are regarded as forming xe2x80x9ctie barsxe2x80x9d tending to maintain a rib-like character in the tread region involved.
xe2x80x9cSipesxe2x80x9d refers to small slots molded into ribs or lugs of a tire that subdivides the tread surface and improves traction characteristics. Sipes tend to close completely in a tire footprint.
xe2x80x9cLugsxe2x80x9d refer to discontinuous radial rows of tread rubber in direct contact with the. road surface.
xe2x80x9cNormal load and inflation pressurexe2x80x9d refers to the specific design inflation pressure and load assigned by the appropriate standards organization for the design rim and service condition for a tire of specific size. Examples of standards are the Tire and Rim Association Manual and the European Tire and Rim Technical Organization.
xe2x80x9cPneumatic tirexe2x80x9d means a laminated mechanical device of generally toroidal shape (usually an open-torus) having beads and a tread and made of rubber, chemicals, fabric and steel or other materials. When mounted on the wheel of a motor vehicle, the tire through its tread provides traction and contains the fluid that sustains the vehicle load.
xe2x80x9cRadialxe2x80x9d and xe2x80x9cradiallyxe2x80x9d are used to mean directions radially toward or away from the axis of rotation of the tire.
xe2x80x9cRibxe2x80x9d means a circumferentially extending strip of rubber on the tread which is defined by at least one circumferential xe2x80x9cwide groovexe2x80x9d and either a second such groove or a lateral edge of the tread, the strip of rubber being laterally undivided by full-depth narrow or wide grooves. xe2x80x9cSidewallxe2x80x9d means that portion of a tire between the tread and the bead.
xe2x80x9cTreadxe2x80x9d means a molded rubber component which, when bonded to a tire casing, includes that portion of the tire that comes into contact with the road when the tire is normally inflated and under normal load.
xe2x80x9cTread widthxe2x80x9d means the arc length of the road contacting tread surface in the axial direction, that is, in a plane parallel to the axis of rotation of the tire.